Quantifying the effects of alteration and acid treatment for whole-rock basalt 40Ar/39Ar geochronology

Abstract

The presence of secondary phases (carbonates, zeolites, clay minerals, etc.) in volcanic rocks and subabyssal intrusive units (sills and dykes) often pose obstacles for high resolution 40Ar/39Ar geochronology as these phases may promote loss of radiogenic argon, production of interfering isotopes, or trapping of excessive atmospheric argon contents. Acid treatment is used to remove alteration products before sample irradiation and analysis. In an effort to assess and quantify the effects of alteration and the removal of alteration products by acid treatment on the accuracy and precision of 40Ar/39Ar analysis, three basalt samples from the Paraná large igneous province previously dated in triplicate without acid treatment were dated again without and with acid treatment. Despite the presence of 14–28% clay minerals in the samples, which results in up to 67% atmospheric component in the total amount of 40Ar extracted from untreated grains, the presence of alteration phases and their removal by acid treatment have no significant role on the accuracy of 40Ar/39Ar step-heating ages. The only quantifiable effect is an ~0.1% increase in age precision for the acid-treated aliquots. The results reveal that effective separation of atmospheric and radiogenic gases in properly designed and sufficiently detailed step-heating experiments successfully obviates deleterious effects associated with some types of alteration phases, and it suffices to produce high-resolution ages. Therefore, acid treatment is a desirable but not essential step in high-resolution 40Ar/39Ar geochronology. Characterizing alteration phases, however, is an essential step as some secondary products (e.g., sericite, albite), if present, will not be leached during acid treatment and will significantly affect the accuracy of geochronological results either with or without acid treatment.